Underwater floating body and installation method thereof

ABSTRACT

The present invention relates to the technical field of ships, and particularly relates to an underwater floating body and an installation method thereof. The floating body includes sub-cabins and pressure resistant cabins, symmetrically arranged sub-cabins are arranged at the left and right sides of the floating body, the sub-cabins are arranged at the front and back sides of the floating body, the buoyant force provided by the sub-cabins at the front side of the floating body is larger than the buoyant force provided by the sub-cabins at the back side of the floating body. According to the underwater floating body and the installation method thereof provided by the present invention, the underwater floating body can arrive at the working water area at one step, thereby saving a large amount of manpower and material resources.

RELATED APPLICATIONS

This application is a United States National Stage Application filed under 35 U.S.0 371 of PCT Patent Application Serial No. PCT/CN2013/079279, filed Jul. 12, 2013, which further claims PCT Patent Application Serial No. PCT/CN2013/073359, filed Mar. 28, 2013, the disclosure of all of which are hereby incorporated by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to the technical field of ships, and particularly relates to an underwater floating body and an installation method thereof

BACKGROUND OF THE INVENTION

As offshore oil and gas resource development equipment, a floating production storage and offloading device (FPSO) is widely applied to oil and gas development under various water depth conditions. For a deepwater floating production storage and offloading device (FPSO), the working position of an underwater floating body thereof is generally in deep water, and the exterior of the floating body bears a very large water pressure. To prevent the floating body structure from being damaged by the larger water pressure, pressure equivalent to the external water pressure must exist in the floating body. A traditional underwater floating body is designed based on a non-pressure resistant structure, the floating body structure is of the non-pressure resistant structure, and the pressure resistance is limited, therefore the non-pressure resistant underwater floating body can be safely and normally installed and work on the premise of bearing no larger pressure. In the traditional underwater floating body, the floating body is inflated to generate a larger pressure in the floating body so as to balance the external water pressure of the floating body, namely the internal pressure of the floating body is increased by inflation when the floating body is located at any underwater position to make the internal pressure of the floating body be equivalent to the external water pressure, so as to protect the structure of the floating body form being damaged by higher water pressure, and for the above reasons, the installation process of the traditional underwater floating body is relatively complicated. In an installation process of the traditional underwater floating body, with the continuous change of water depth, the internal pressure of the floating body needs to be continuously adjusted, and meanwhile the posture of the floating body needs to be continuously adjusted. That is, with the increase of the water depth, the external water pressure of the floating body increases continuously, and to prevent the floating body from bearing overlarge pressure, the floating body needs to be continuously inflated to increase the internal pressure of the floating body to balance the external water pressure. The pressure bearing capability of the floating body structure is limited, thus the inflation process of the floating body must be performed segment by segment, namely pressure and balance must be adjusted once whenever reaching a certain water level, for example, the working water depth of a certain underwater floating body is about 300 m, and if this operation is performed once every 5 m (set according to the pressure resistance of the floating body), then the operations of pressure adjustment and underwater posture adjustment of the floating body need be performed for dozens of times in the entire installation process. Moreover, the installation process of the traditional underwater floating body is completed underwater by a control system under the help of an underwater operating system (ROV), thus the installation process is difficult to achieve.

SUMMARY OF THE INVENTION

The technical problem to be solved in the present invention is to provide an underwater floating body which can be installed underwater at one step and can save a large amount of manpower and material resources, and an installation method thereof

To solve the above technical problem, the present invention provides an underwater floating body, including sub-cabins and pressure resistant cabins. The sub-cabins are arranged at the left and right sides of the floating body, and the sub-cabins at the left and right sides of the floating body provide the same buoyant force. The sub-cabins are arranged at the front and back sides of the floating body, the buoyant force provided by the sub-cabins at the front side of the floating body is larger than the buoyant force provided by the sub-cabins at the back side of the floating body, or the buoyant force provided by the sub-cabins at the back side of the floating body is larger than the buoyant force provided by the sub-cabins at the front side of the floating body. The pressure resistant cabins penetrate through the sub-cabins and are fixedly connected with the bulkheads of the sub-cabins. The buoyant center of the floating body and the gravity center of the floating body are located on the same vertical line, and the position of the buoyant center of the floating body is higher than the position of the gravity center of the floating body. An inflation valve is arranged on each pressure resistant cabin. A ventilating system and a water supply system are arranged on each sub-cabin.

Further, the buoyant force provided by the sub-cabins at the front side of the floating body is larger than the buoyant force provided by the sub-cabins at the back side of the floating body, or the buoyant force provided by the sub-cabins at the back side of the floating body is larger than the buoyant force provided by the sub-cabins at the front side of the floating body.

The number of the sub-cabins at the front side of the floating body is larger than the number of the sub-cabins at the back side of the floating body, or the number of the sub-cabins at the back side of the floating body is larger than the number of the sub-cabins at the front side of the floating body.

Further, at least one pressure resistant cabin is provided.

Further, the underwater floating body further includes a posture monitoring system and a controller. The posture monitoring system, the ventilating system and the water supply system are respectively connected with the controller. The posture monitoring system is used for monitoring the position of the floating body and monitoring that the floating body is at a balanced state or an inclined state, and when the floating body is at the inclined state, the controller controls the ventilating system to inflate the sub-cabin at the downward inclined end of the floating body until the floating body is not inclined any more.

Further, the posture monitoring system is composed of four position sensors. The four position sensors are respectively installed on the four corners on the surrounding of the floating body; the four position sensors are respectively connected with the controller.

The present invention further provides an installation method of the underwater floating body, including: respectively inflating the pressure resistant cabins and filling water into the sub-cabins; putting the underwater floating body in a working water area; inflating the sub-cabins to discharge water in the sub-cabins, so as to enable the sub-cabins to generate an upward positive buoyant force.

Further, the respectively inflating the pressure resistant cabins and filling water into the sub-cabins includes: connecting a water surface inflation system with the inflation valve on each pressure resistant cabin, inflating the pressure resistant cabin via the water surface inflation system, and closing the inflation valve on the pressure resistant cabin and the water surface inflation system when the gas pressure in the pressure resistant cabin is consistent with the water pressure of the working water area; opening the ventilating system on each sub-cabin to keep the ventilating system on each sub-cabin at a normal pressure state; and filling water into each sub-cabin through the water supply system.

Further, the putting the underwater floating body in a working water area includes: putting the pressure resistant cabins and the sub-cabins in water until the pressure resistant cabins and the sub-cabins are completely submerged in water; using a hauling system to haul the floating body downwards; monitoring the posture of the entire floating body via a sensor on the hauling system, and using the hauling system to straighten the floating body when the floating body is inclined; stopping the hauling action of the hauling system after the underwater floating body arrives at the working water area.

Further, the inflating the sub-cabins to discharge a part of water in the sub-cabins, so as to enable the sub-cabins to generate an upward positive buoyant force includes: controlling the ventilating system to inflate each sub-cabin through the controller to discharge a part of water in each sub-cabin; closing the ventilating system on each sub-cabin after each sub-cabin is inflated, so as to enable each sub-cabin to provide the upward positive buoyant force.

Further, the method further includes: monitoring the position of the floating body via the four position sensors distributed on the four corners on the surrounding of the floating body after inflating each sub-cabin, monitoring that the floating body is at the balanced state or the inclined state, and when the floating body is at the inclined state, controlling the ventilating system through the controller to inflate the sub-cabin at the downward inclined end of the floating body until the floating body is not inclined any more.

According to the underwater floating body provided by the present invention, the maximum buoyant force capable of being provided by the sub-cabins arranged at the left side of the floating body is equal to the maximum buoyant force capable of being provided by the sub-cabins arranged at the right side of the floating body, therefore the left and right sides of the floating body can be kept at an approximately stable state. The maximum buoyant force capable of being provided by the sub-cabins arranged at the front side of the floating body is different from the maximum buoyant force capable of being provided by the sub-cabins arranged at the back side of the floating body, therefore a deep sea pipeline can be loaded according to different gravities on different sides of the deep sea pipeline. Meanwhile, the position of the buoyant center and the position of the gravity center of the floating body are located on the same vertical line, and the position of the buoyant center is higher than the position of the gravity center, so that the entire floating body can be kept at a stable state when at work.

The pressure resistant cabins can meet the requirements of bearing larger pressure, after the pressure resistant cabins are fully inflated, in a submerging process of the floating body, the pressure resistant cabins provide an upward buoyant force so as to overcome the gravity of the floating body itself to stably submerge the floating body; since the resultant force of the upward buoyant force provided by the pressure resistant cabins and the downward gravity of the floating body is smaller, the floating body is basically at the stable state, thereby reducing the force application strength of the hauling system on the floating body and reducing the requirements on the structural strength at the connecting sites with the hauling system on the sub-cabins. After being inflated in the working water area, the sub-cabins provide the upward positive buoyant force to ensure the normal work of the underwater floating body.

According to the installation method of the underwater floating body provided by the present invention, the posture adjustment process is simple and controllable, and the underwater floating body can arrive at a preset water depth at one step without being gradually adjusted during installation, thereby improving the installation efficiency and saving a large amount of manpower and material resources. In the entire submerging process of the floating body, no inflation or deflation operation is carried out, and the pressure in the pressure resistant cabins is always at a self-balancing state. Meanwhile, the fine tuning of the posture of the floating body in the entire submerging process is completely achieved by the hauling system, so that the adjustment is convenient. The operations in the entire installation process are completed by a water surface control system, and no underwater operation is carried out. Therefore, the underwater floating body provided by the present invention can be installed without the help of the underwater operating system (ROV), so that the installation cost is greatly reduced and the installation controllability is stronger. After the floating body enters the working state, either the sub-cabins or the pressure resistant cabins nearly bear no pressure, thereby prolonging the service lives of the pressure resistant cabins and the sub-cabins.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a structure of an underwater floating body provided by an embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

See FIG. 1, the embodiment of the present invention provides an underwater floating body. The underwater floating body is mainly composed of sub-cabins 1, pressure resistant cabins 2, a posture monitoring system and control equipment. At first, the structure of each sub-cabin 1 is described, the sub-cabins 1 are formed by welding plates with different specifications, specifically: plates with the same material and thickness are adopted, different plates are welded together to form a plurality of cabins, and the plates are high-strength and corrosion-resistant steel plates. All the cabins can be rectangles or squares in shape, and all shapes conforming to the design idea of the present invention are encompassed within the protection scope of the present invention. Each cabin is a relatively independent closed space, and in an actual manufacturing process, a plate with a larger area is used as the bottom plate of all the cabins. One cabin is called a sub-cabin 1. In the embodiment, all the sub-cabins 1 are arranged closely, two adjacent sub-cabins 1 share a bulkhead, all the sub-cabins 1 are distributed to form a square integral structure, the square integral structure formed by all the sub-cabins 1 is eudipleural, and the symmetrical arrangement of the sub-cabins 1 is an important means for keeping the balance of the entire floating body. In the embodiment, two rows of sub-cabins 1 are distributed on the front side (namely the A side in FIG. 1) of the square integral structure, one row of sub-cabins 1 is respectively distributed on the left side, the right side and the back side (namely the B side in FIG. 1) of the square integral structure, the maximum buoyant force capable of being provided by the sub-cabins 1 on the front side of the square entirety (namely the front side of the floating body) is larger than the maximum buoyant force capable of being provided by the sub-cabins 1 on the back side of the square entirety (namely the back side of the floating body), or the maximum buoyant force capable of being provided by the sub-cabins 1 on the back side of the square entirety (namely the back side of the floating body) is larger than the maximum buoyant force capable of being provided by the sub-cabins 1 on the front side of the square entirety (namely the front side of the floating body). In the embodiment, the maximum buoyant force capable of being provided on the front side and the back side of the floating body can be determined according to the number of the sub-cabins on the front side and the back side of the floating body, and for the sub-cabins with the same specification, the larger the number is, the larger the maximum buoyant force capable of being provided is. The eudipleural but fore-and-aft asymmetrical structures of the sub-cabins 1 are designed according to the application of the floating body, the floating body is mainly used for supporting a subsea oil pipeline, the oil pipeline extends all the way from the seabed to the sea surface, the oil pipeline extending from the seabed is fixed on the front end of the square entirety (namely the front end of the floating body) and extends to the sea surface through the back end of the square entirety (namely the back end of the floating body); since the length of the end of the oil pipeline extending from the seabed is larger than the length of the end extending to the sea surface, the weight of the end of the oil pipeline extending from the seabed is larger than the weight of the end extending to the sea surface; more sub-cabins 1 need to be designed on the front end of the square entirety to provide a larger buoyant force to bear the end with larger weight on the oil pipeline. The internal spaces of the sub-cabins 1 and the internal spaces of the pressure resistant cabins 2 are not communicated, namely, gas or liquid (e.g., water) cannot circulate in the pressure resistant cabins 2 and the sub-cabins 1. A ventilating system and a water supply system are respectively installed on each sub-cabin 1, the ventilating system is mainly composed of inflation equipment for industrial use and is used for inflating the sub-cabin 1 to discharge water; the water supply system is mainly composed of a water supply pipeline and is used for filling water into and discharging water from the sub-cabin 1. The structure of each pressure resistant cabin 2 is described as follows, the pressure resistant cabin 2 is of an elliptic cylindrical structure (namely, the middle of the pressure resistant cabin 2 is cylindrical and the two ends are respectively hemispherical), and in the embodiment of the present invention, the pressure resistant cabin 2 is made of high-strength pressure resistant steel. An inflation valve is arranged on each pressure resistant cabin 2, the inflation valve is an inflation inlet of the pressure resistant cabin, the pressure resistant cabin is inflated by common industrial high pressure inflation equipment through the inflation valve, and the inflation valve is closed after inflation to prevent gas leak in the inflation valve. 5 pressure resistant cabins 2 are installed on the underwater floating body provided by the embodiment of the present invention, the pressure resistant cabins 2 penetrate through one or more sub-cabins 1, and the pressure resistant cabins 2 are fixedly connected with the bulkheads of the sub-cabins by welding. Since all the sub-cabins 1 are divided into 5 rows to form the square integral structure, in the embodiment of the present invention, one pressure resistant cabin 2 penetrates through each row of sub-cabins 1, namely the distribution direction of the 5 pressure resistant cabins 2 is consistent with the distribution direction of the 5 rows of sub-cabins 1, then the distributed 5 pressure resistant cabins 2 are eudipleural, and the symmetrical distribution of the pressure resistant cabins is an important design for keeping the balance of the entire floating body structure as well. In the embodiment, the sub-cabins 1 are not communicated with the pressure resistant cabins 2, the floating body structure will be illustrated below through a group of more specific data, for example, according to engineering demands, the total weight of the floating body is about 7 tons, and the positive buoyant force required to be provided by the floating body is about 3 tons. See FIG. 1, in the embodiment, 5 pressure resistant cabins 2 are designed, which are respectively marked as 1, 2, 3, 4, 5; the parameter settings of the pressure resistant cabins 2 are as shown in table 1:

TABLE 1 Serial number of pressure Thick- Single Total resistant Number Length ness Radius mass mass cabin (piece) (m) (m) (m) (t) (t) 1-2 2 9.5 0.004 0.25 0.49298 0.98596 3-4 2 7.5 0.004 0.25 0.394384 0.788768 5 1 5.5 0.004 0.25 0.295788 0.295788 Total 2.070516

It can be calculated by the parameters of the pressure resistant cabins as shown in table 1 that, the total mass of the 5 pressure resistant cabins is 2.070516 tons, the buoyant force respectively provided by the 5 pressure resistant cabins 2 and the total buoyant force provided by the 5 pressure resistant cabins 2 are calculated as follows, as shown in table 2:

TABLE 2 Serial number of Single Provided pressure Thick- displace- buoyant resistant Number Length ness Radius ment force cabin (piece) (m) (m) (m) (t) (t) 1-2 2 9.5 0.004 0.25 1.979966 3.959933 3-4 2 7.5 0.004 0.25 1.577261 3.154523 5 1 5.5 0.004 0.25 1.174556 1.174556 Total 8.289011

It can be seen from the calculation result provided by table 2 that, the total buoyant force capable of being provided by the 5 pressure resistant cabins 2 is 8.289011 tons. Since the weight of the entire floating body is about 7 tons, the total weight of the 5 pressure resistant cabins 2 is 2.070516 tons, the difference between the weight of the entire floating body and the weight of the 5 pressure resistant cabins 2 is about 5 tons, then in a design process, the total mass of all structures (for example, the sub-cabins 1, other facilities on the floating body, etc.) excluding the 5 pressure resistant cabins 2 on the floating body needs to be controlled at about 5 tons. Since the total mass (about 7 tons) of the floating body is slightly smaller than the total buoyant force (8.289011 tons) provided by the 5 pressure resistant cabins 2, the submerging stability of the floating body can be guaranteed under the hauling effect of the hauling system. In the design process of the floating body, the buoyant center of the floating body and the gravity center of the floating body are located on the same vertical line, and the buoyant center of the floating body is slightly higher than the gravity center of the floating body. It is set that the position of the buoyant center is (xB, yB, zB) and the position of the gravity center is (xG, yG, zG), therefore the position relationship of the gravity center and the buoyant center needs to satisfy: x_(G)=x_(B)=0, y_(G)=y_(B) and z_(B)≧z_(G)≧0. In a process of determining the buoyant center of the floating body and the gravity center of the floating body, the position of the buoyant center is calculated at first, then the structures and sizes of the sub-cabins 1 and other facilities on the floating body are adjusted according to the position of the buoyant center to adjust the position of the gravity center, so as to locate the gravity center of the floating body and the buoyant center of the floating body on the same vertical line and make the buoyant center of the floating body be slightly higher than the gravity center of the floating body. It should be noted that, when calculating the coordinates of the gravity center, the weights of the 5 pressure resistant cabins 2 and the floating body structure (for example, the sub-cabins 1, other facilities on the floating body, etc.) need to be considered at the same time, namely the gravity center of the floating body is the gravity center of the entirety formed by the 5 pressure resistant cabins 2 and the floating body structure (for example, the sub-cabins 1, other facilities on the floating body, etc.). The posture monitoring system is composed of four position sensors, the four position sensors are respectively distributed on the four corners of the floating body, the controller calculates according to position signals fed back by the four position sensors to obtain the posture angle of the floating body and judges whether the floating body is at a balanced state or a certain inclined state, the posture monitoring system is connected with control equipment, and the control equipment is connected with the ventilating system and the water supply system of each sub-cabin 1. The posture monitoring system monitors the position of the floating body and monitors that the floating body is at the balanced state or the inclined state, when the floating body is at the inclined state, the controller judges which end of the floating body is inclined downwards according to the position information of the floating body obtained by the posture monitoring system and controls the ventilating system to inflate the sub-cabins 1 at the downward inclined end until the floating body is not inclined any more.

The embodiment of the present invention further provides an installation method of the underwater floating body as shown in FIG. 1 to FIG. 3, including the following steps:

step 10: respectively inflating the pressure resistant cabins 2 and filling water into the sub-cabins 1, specifically: connecting a water surface inflation system with the inflation valves on the pressure resistant cabins 2; opening the inflation valves on the pressure resistant cabins 2, inflating the pressure resistant cabins 2 via the water surface inflation system, and closing the water surface inflation system and the inflation valves on the pressure resistant cabins when the gas pressure in each pressure resistant cabin 2 is consistent with the water pressure of a working water area; opening the ventilating system on each sub-cabin 1 to keep the ventilating system at a normal pressure state; opening the water supply system on the sub-cabin 1 to fill each sub-cabin 1 with water;

step 20: putting the underwater floating body in the working water area, specifically: putting the underwater floating body in water until the pressure resistant cabins 2 and the sub-cabins 1 are completely submerged in water; opening the water supply systems on all the sub-cabins 1 to communicate water at the outside of the sub-cabins 1 with the internal spaces of the sub-cabins 1, and using a hauling system to haul the floating body downwards; stopping the hauling action of the hauling system after the underwater floating body arrives at the working water area;

step 30: inflating the sub-cabins 1 to discharge a part of water in the sub-cabins 1, so as to enable the sub-cabins 1 to generate an upward positive buoyant force, specifically: determining the total buoyant force needing to be provided by the sub-cabins 1, calculating the total displacement of all the sub-cabins 1 according to the total buoyant force needing to be provided by the sub-cabins 1, and determining the total inflation amount of all the sub-cabins 1 according to the total displacement of all the sub-cabins 1; after determining the total inflation amount of all the sub-cabins 1, averagely distributing the total inflation amount to all the sub-cabins 1, determining the inflation amount of each sub-cabin 1, opening the ventilating systems of the sub-cabins 1 through a water surface controller to start inflating to discharge a part of water in the sub-cabins 1, so as to enable the sub-cabins 1 to provide the upward buoyant force; monitoring the position of the floating body through the posture monitoring system, monitoring that the floating body is at the balanced state or the inclined state, and when the floating body is at the inclined state, controlling the ventilating system through the controller to inflate the sub-cabin 1 at the downward inclined end of the floating body until the floating body is not inclined any more; closing the ventilating systems of the sub-cabins 1 after each sub-cabin 1 is inflated, to enable each sub-cabin 1 to provide an upward positive buoyant force. The water surface inflation system includes a gas source and a controller for controlling inflation of the gas source, which belong to the prior art. The calculation method of the inflation amounts of all the sub-cabins 1 will be illustrated below through a group of more specific data: illustration for calculation is given by taking it as an example that nitrogen needs to be filled in the sub-cabins 1 to provide a buoyant force of 3 t. It is assumed that requirements are satisfied when the nitrogen mole number is n, and it is known that the molar mass of N2 is M=28 g/mol; a gas state equation is PV=nRT; wherein P refers to pressure, and the calculated pressure is P=250*1026.05*9.8=2.5138 MPa; V refers to the volume of the corresponding nitrogen; R=8.314; T=283.15 (Fahrenheit, the underwater temperature is set as)10°; the volume of discharged water is V=nRT/P; the mass of the discharged water is ml=rho* nRT/P (rho refers to water density); the mass of filled nitrogen is m2=0.028 n; the increased buoyant force is ml-m2=3000 Kg; it is calculated that n=32158, the nitrogen mass is 90.0424 Kg and the nitrogen volume is 3.0115 m3. Whether the pressure resistant cabins are completely submerged in water is verified after the nitrogen is filled to discharge water. The nitrogen volume is 3.0115m3, and the decreased height of the water level in the floating body is 3.0115/90=0.0335 m. It can be calculated according to the position of the pressure resistant cabin that the distance from the top of the pressure resistant cabin to the top of the floating body is 0.15, obviously, after water with corresponding volume is discharged, the pressure resistant cabin is still completely submerged in water, and it indicates that the above-mentioned calculation is correct. Namely, about 90 Kg of nitrogen is filled in all the sub-cabins 1 to provide the buoyant force of 3 t for the floating body, and the 90 Kg of nitrogen is averagely distributed to all the sub-cabins 1 to obtain the inflation amount of each sub-cabin 1. In the embodiment of the present invention, the unit of displacement is ton, and the unit of the inflation amount is cubic meter.

The working principle of the floating body is analyzed and illustrated below: before the floating body is submerged in water, the pressure resistant cabins 2 are inflated to make the gas pressure in the pressure resistant cabins 2 be equivalent to the water pressure of the working water area, since the pressure resistant cabins 2 themselves have equivalent pressure bearing capability, the pressure resistant cabins can bear the internal gas pressure. With the increase of the submergence depth of the floating body, the external water pressure increases to gradually balance the gas pressure in the pressure resistant cabins 2 until arriving at the working water area, and the external water pressure is basically equivalent to the gas pressure in the pressure resistant cabins 2, thus it can be approximately considered that the pressure resistant cabins 2 nearly bear no pressure at a preset water depth. For the internal spaces of the sub-cabins 1, due to the existence of the water supply systems, the pressure in the sub-cabins 1 is always equal to the external water pressure, namely the bulkheads of the sub-cabins 1 nearly bear no pressure. In the entire installation process and the working process after installation of the floating body, the structures of the floating body can satisfy the pressure requirements. In the submerging process of the floating body, the floating body is applied with the buoyant force and the gravity, and the posture balance of the floating body can be guaranteed as long as the gravity center and the buoyant center of the floating body are guaranteed to be located on the same vertical line. According to the underwater floating body provided by the embodiment of the present invention, the buoyant force of the floating body in the submerging process is only provided by the pressure resistant cabins 2, and during design, the sizes and the installation positions of the pressure resistant cabins 2 can be controlled to control the position of the buoyant center of the entire floating body. During design, it is guaranteed that the gravity center and the buoyant center of the entire floating body system are located on the same vertical line when the sub-cabins 1 are filled with water, and the position of the buoyant center is higher than the position of the gravity center, the posture of the floating body is kept balanced all the way in the entire submerging process. After the floating body arrives at the preset water depth, most of the positive buoyant force of the floating body needs to be provided by the internal spaces of the sub-cabins 1. The sub-cabins 1 are inflated to discharge a part of water in the sub-cabins 1, and the weight of the discharged water is just equal to the positive buoyant force, in this way, the sub-cabins 1 can provide the positive buoyant force satisfying the working requirements. The specific method of controlling the displacement of each sub-cabin 1 is as follows: step 110. the necessary total displacement is calculated according to requirements of the floating body on the positive buoyant force, the total inflation amount of all the sub-cabins 1 is calculated according to the total displacement, the total inflation amount is averagely distributed to all the sub-cabins 1, and the displacement of each sub-cabin 1 is determined. According to the requirements of posture balance of the floating body, the sum of the displacements of the sub-cabins is slightly smaller than the necessary total displacement. Step 220. after each sub-cabin 1 is inflated according to the inflation amount thereof, the four position sensors monitor the position of the floating body, the controller calculates according to position signals fed back by the four position sensors to obtain the posture angle of the floating body and judges that the floating body is at a balanced state or a certain inclined state, when the floating body is at the inclined state, the controller judges which end of the floating body is inclined downwards according to the position information of the floating body obtained by the posture monitoring system and controls the ventilating system to inflate the sub-cabin 1 at the downward inclined end until the floating body is not inclined. Step 110 can be preset before the floating body is submerged to ensure the submerging safety of the floating body. The operation principle of water discharge by inflation of the sub-cabins 1 is as follows: the gas pressure in the sub-cabins 1 is increased by inflation to be larger than the external water pressure, and water in the sub-cabins 1 can be automatically discharged through the water supply systems under the effect of pressure difference. After a part of water is discharged, the gas spaces in the sub-cabins 1 become larger, the gas pressure is reduced, when the gas pressure in sub-cabins 1 is reduced to be smaller than the external water pressure, water will enter the sub-cabins 1 through the water supply systems to reduce the gas spaces in the sub-cabins 1 so as to increase the gas pressure. The above process is repeated to eventually reach a dynamic balance. The displacements in the sub-cabins 1 can be converted into inflation amounts, after the inflation amount of each sub-cabin 1 is calculated, the posture balance of the floating body can be completely controlled through the inflation amount, so as to keep the floating body at a stable working state.

The embodiment of the present invention has the following beneficial effects:

1. the maximum buoyant force capable of being provided by the sub-cabins arranged at the left side of the floating body is equal to the maximum buoyant force capable of being provided by the sub-cabins arranged at the right side of the floating body, therefore the left and right sides of the floating body can be kept at an approximately stable state. The maximum buoyant force capable of being provided by the sub-cabins arranged at the front side of the floating body is different from the maximum buoyant force capable of being provided by the sub-cabins arranged at the back side of the floating body, therefore a deep sea pipeline can be loaded according to different gravities on different sides of the deep sea pipeline. Meanwhile, the position of the buoyant center and the position of the gravity center of the floating body are located on the same vertical line, and the position of the buoyant center is higher than the position of the gravity center, so that the entire floating body can be kept at a stable state when at work. The pressure resistant cabins can meet the requirements of bearing larger pressure, in a submerging process of the floating body, the fully inflated pressure resistant cabins provide an upward buoyant force so as to overcome the gravity of the floating body itself to stably submerge the floating body. Since the resultant force of the upward buoyant force provided by the pressure resistant cabins and the downward gravity of the floating body is smaller, the floating body is basically at the stable state, thereby reducing the force application strength of the hauling system on the floating body and reducing the requirements on the structural strength at the connecting sites with the hauling system on the sub-cabins.

2. The pressure resistant cabins are designed to the elliptic cylindrical structures, thereby having higher pressure resistance.

3. By adopting the installation solution in the present invention, only two larger posture adjustment operations are needed and can be guided by accurate calculation, so that the posture adjustment process is simple and controllable, and the underwater floating body can arrive at a preset water depth at one step without being gradually adjusted during installation, thereby improving the installation efficiency and saving a large amount of manpower and material resources.

4. In the entire submerging process of the floating body, no inflation or deflation operation is carried out, and the pressure in the floating body is always at a self-balancing state. Meanwhile, the fine tuning of the posture of the floating body in the entire submerging process is completely achieved by the hauling system, so that the adjustment is convenient.

5. The operations in the entire installation process are completed by the water surface control system, and no underwater operation is carried out. Therefore, the underwater floating body provided by the present invention can be installed without the help of the underwater operating system (ROV), so that the installation cost is greatly reduced and the installation controllability is stronger.

6. After the floating body enters the working state, either the sub-cabins or the pressure resistant cabins nearly bear no pressure, thereby prolonging the service lives of the pressure resistant cabins and the sub-cabins.

Finally, it should be noted that, the foregoing implementations are merely used for illustrating the technical solutions of the present invention, rather than limiting, although the present invention has been described in detail with reference to examples, those of ordinary skill in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention, without departing from the spirit and scope of the technical solutions of the present invention, and these modifications or equivalent substitutions are encompassed within the scope of the claims of the present invention. 

1. An underwater floating body, comprising sub-cabins (1) and pressure resistant cabins (2); the sub-cabins (1) are arranged at the left and right sides of the floating body, and the sub-cabins at the left and right sides of the floating body provide the same buoyant force; the sub-cabins are arranged at the front and back sides of the floating body, the buoyant force provided by the sub-cabins (1) at the front side of the floating body is larger than the buoyant force provided by the sub-cabins (1) at the back side of the floating body, or the buoyant force provided by the sub-cabins (1) at the back side of the floating body is larger than the buoyant force provided by the sub-cabins (1) at the front side of the floating body; the pressure resistant cabins (2) penetrate through the sub-cabins (1) and are fixedly connected with the bulkheads of the sub-cabins (1); the buoyant center of the floating body and the gravity center of the floating body are located on the same vertical line, and the position of the buoyant center of the floating body is higher than the position of the gravity center of the floating body; an inflation valve is arranged on each pressure resistant cabin (2); a ventilating system and a water supply system are arranged on each sub-cabin (1).
 2. The underwater floating body of claim 1, wherein the buoyant force provided by the sub-cabins (1) at the front side of the floating body is larger than the buoyant force provided by the sub-cabins (1) at the back side of the floating body, or the buoyant force provided by the sub-cabins (1) at the back side of the floating body is larger than the buoyant force provided by the sub-cabins (1) at the front side of the floating body, comprising: the number of the sub-cabins (1) at the front side of the floating body is larger than the number of the sub-cabins (1) at the back side of the floating body, or the number of the sub-cabins (1) at the back side of the floating body is larger than the number of the sub-cabins (1) at the front side of the floating body.
 3. The underwater floating body of claim 1, wherein at least one pressure resistant cabin (2) is provided.
 4. The underwater floating body of claim 1, further comprising a posture monitoring system and a controller; the posture monitoring system, the ventilating system and the water supply system are respectively connected with the controller; the posture monitoring system is used for monitoring the position of the floating body and monitoring that the floating body is at a balanced state or an inclined state, and when the floating body is at the inclined state, the controller controls the ventilating system to inflate the sub-cabin (1) at the downward inclined end of the floating body until the floating body is not inclined any more.
 5. The underwater floating body of claim 4, wherein the posture monitoring system is composed of four position sensors; the four position sensors are respectively installed on the four corners on the surrounding of the floating body; the four position sensors are respectively connected with the controller.
 6. An installation method of the underwater floating body of claim 5, comprising: respectively inflating the pressure resistant cabins (2) and filling water into the sub-cabins (1); putting the underwater floating body in a working water area; inflating the sub-cabins (1) to discharge water in the sub-cabins (1), so as to enable the sub-cabins (1) to generate an upward positive buoyant force.
 7. The installation method of claim 6, wherein the respectively inflating the pressure resistant cabins (2) and filling water into the sub-cabins (1) comprises: connecting a water surface inflation system with the inflation valve on each pressure resistant cabin (2), inflating the pressure resistant cabin (2) via the water surface inflation system, and closing the inflation valve on the pressure resistant cabin (2) and the water surface inflation system when the gas pressure in the pressure resistant cabin (2) is consistent with the water pressure of the working water area; opening the ventilating system on each sub-cabin (1) to keep the ventilating system on each sub-cabin (1) at a normal pressure state; filling water into each sub-cabin (1) through the water supply system.
 8. The installation method of claim 7, wherein the putting the underwater floating body in a working water area comprises: putting the pressure resistant cabins (2) and the sub-cabins (1) in water until the pressure resistant cabins (2) and the sub-cabins (1) are completely submerged in water; using a hauling system to haul the floating body downwards; stopping the hauling action of the hauling system after the underwater floating body arrives at the working water area.
 9. The installation method of claim 8, wherein the inflating the sub-cabins (1) to discharge a part of water in the sub-cabins (1), so as to enable the sub-cabins (1) to generate an upward positive buoyant force comprises: controlling the ventilating system to inflate each sub-cabin (1) through the controller to discharge a part of water in each sub-cabin (1); closing the ventilating system on each sub-cabin (1) after each sub-cabin (1) is inflated, so as to enable each sub-cabin (1) to provide the upward positive buoyant force.
 10. The installation method of claim 9, further comprising: monitoring the position of the floating body via the four position sensors distributed on the four corners on the surrounding of the floating body after inflating each sub-cabin (1), monitoring that the floating body is at the balanced state or the inclined state, and when the floating body is at the inclined state, controlling the ventilating system through the controller to inflate the sub-cabin (1) at the downward inclined end of the floating body until the floating body is not inclined any more. 